A kind of light-emitting element, organic electroluminescence elements (henceforth “organic EL elements”) can be made to emit light in desired colors by appropriate choice of light-emitting materials. By proper combination of light-emitting materials, it is possible even to obtain white light, and thus organic EL elements are expected to find uses as backlights, i.e. light-emitting devices, in liquid crystal display devices and, for their low power consumption combined with high luminance, as illuminating light sources to supplant fluorescent and tungsten lamps.
When used as a backlight in a liquid crystal display device, an organic EL element is arranged on the back of a liquid crystal panel, and image display is achieved as a result of the light emitted from the organic EL element being modulated by the liquid crystal layer of the liquid crystal panel. Other than organic EL elements, widely used as backlights are light-emitting diodes (henceforth “LEDs”) and cathode lamps. Since LEDs are point light sources and cathode lamps are line light sources, however, the light emitted from them needs to be guided, by use of a light guide plate, to the back of a liquid crystal panel. This leads to disadvantages such as large device thickness and uneven brightness. By contrast, since organic EL elements are plane light sources, they do not require a light guide plate, and radiate uniform light with even brightness.
Now, with reference to FIG. 20, the structure of a common organic EL element will be described. An organic EL element 110 has a first electrode 112, a light-emitting layer 114, and a second electrode 115 provided in this order on a transparent substrate 111 formed of glass or the like. The first electrode 112 is a film of ITO (indium tin oxide) or IZO (indium zinc oxide) formed by sputtering or the like on the transparent substrate 111, and is transparent. The light-emitting layer 114 is a film of an organic light-emitting material formed on the first electrode 112. The second electrode 115 is a film of a metal formed by vacuum deposition or the like on the light-emitting layer 114.
When a voltage is applied between the first electrode 112 and the second electrode 115, light emission occurs in the light-emitting layer 114 as a result of recombination of holes injected from the first electrode 112, functioning as an anode, and electrons injected from the second electrode 115, functioning as a cathode. The light is transmitted through the first electrode 112 and the transparent substrate 111, so that to an observer the organic EL element 110 appears to emit light. By controlling the dopant added to the light-emitting layer 114, the wavelength of the light emitted can be varied.
To obtain white light from the organic EL element 110, light of two wavelengths is mixed by some methods (two-wavelength type), and light of three wavelengths is mixed by other methods (three-wavelength type).
Examples of Two-Wavelength Type Methods Include:
1. a method according to which the light-emitting layer 114 is made to emit blue light and also orange light, i.e. the color complementary to blue (EL+EL);
2. a method according to which, in the vicinity of the light-emitting layer 114, provided as a single layer and emitting blue light, a fluorescent or phosphorescent member is placed that by absorbing the blue light fluoresces or phosphoresces in orange (EL+PL).
When the EL+EL method noted at 1. above is adopted, separate layers may be provided to emit light of different colors, or alternatively a single layer may be doped with dopants for different colors so that white light is emitted as a result of mixture of light of two colors. Here, EL is short for electroluminescence and PL is short for photoluminescence.
Examples of Three-Wavelength Type Methods Include:
1. a method according to which the light-emitting layer 114 is made to emit blue, green, and red light (EL+EL+EL);
2. a method according to which, in the vicinity of the light-emitting layer 114, provided as a single layer and emitting blue light, a fluorescent or phosphorescent member is placed that by absorbing the blue light fluoresces or phosphoresces in green and red (EL+PL+PL); and
3. a method according to which, in the vicinity of the light-emitting layer 114, provided as a single layer and emitting ultraviolet rays, a fluorescent or phosphorescent member is placed that by absorbing the ultraviolet rays fluoresces or phosphoresces in blue, green, and red (EL+PL+PL+PL).
When the EL+EL+EL method noted at 1. above is adopted, separate layers may be provided to emit light of different colors, or alternatively a single layer may be doped with dopants for different colors so that white light is emitted as a result of mixture of light of three colors.
For enhanced color rendering, for example, a four-wavelength type method is possible that uses, in addition to blue, green, and red light, cyan (blue-green) light.
In general, depending on the dopant added to it, the light-emitting layer 114 may fluoresce or phosphoresce. In the current state of the art, fluorescence and phosphorescence in different colors exhibit tendencies as shown in Table 1 in terms of light emission efficiency and light emission lifetime. In Table 1, “Good” denotes a level inferior to “Excellent” or “Good” but still not unacceptable in practical terms.
TABLE 1TypeColorEfficiencyLifetimeFluorescenceBlueFairFairGreenFairExcellentRedPoorPoorPhosphorescenceBlueGoodUnacceptableGreenExcellentGoodRedGoodExcellent
As will be understood from Table 1, both fluorescence and phosphorescence in green are on the whole superior to those in other colors in terms of light emission efficiency and light emission lifetime; however, fluorescence in blue is inferior to that in green in terms of light emission lifetime, and so is fluorescence in red in terms of light emission efficiency and light emission lifetime; phosphorescence in blue has a far shorter lifetime than that in other colors.
In an organic EL element, the light emission luminance is approximately proportional to the current density; thus the light emission luminance can be increased by increasing the applied voltage, thereby increasing the injected current, and thereby increasing the current density. On the other hand, since the light emission lifetime deteriorates in inverse proportion to the current density raised to the power of 1.5 to 2, there is demand for efficient use of the emitted light with the current density, and hence the applied voltage, lowered as much as possible.
In general, a backlight for a liquid crystal display device needs to have a front luminance of about 2 000 to 4 000 cd/m2. Inconveniently, however, in an organic EL element as a white light source, uneven light emission efficiency and uneven light emission lifetime among different colors as described above lead to, when the current density is so set as to promise a satisfactory light emission lifetime, a front luminance as low as about 1 000 to 1 500 cd/m2.
Inconveniently, with organic EL elements with a layer structure like that of the organic EL element 110 shown in FIG. 20, the proportion of, of the light generated in the transparent substrate 111, the part that manages to come out through the transparent substrate 111—the proportion called light extraction efficiency—is extremely small, being about 15 to 20%. This is believed to be due to the following facts: the light that is incident on the interface between the transparent substrate 111 and air—the interface called the observation surface—at incidence angles equal to or larger than the critical angle is totally reflected, and thus cannot come out through the observation surface of the transparent substrate 111; likewise, also at the interface between the transparent substrate 111 and the first electrode 112 and at the interface between the first electrode 112 and the light-emitting layer 114, the light incident at large incidence angles is totally reflected and thereby guided inside the first electrode 112 and the light-emitting layer 114, thus unable to come out through the observation surface of the transparent substrate 111 but escaping to the side of the organic EL element 110.
As one way to enhance the light extraction efficiency, Patent Document 1 listed below proposes a method according to which the light exit surface of the transparent substrate is formed in the shape of a large number of convex lenses so that the transparent substrate has a light-condensing ability; Patent Document 2 listed below proposes a method according to which a mirror is provided on the side of the light-emitting layer opposite from its light extraction side. Inconveniently, in the small element it is difficult to form the shape of convex lenses, and with the thin light-emitting layer it is difficult to shape it into a taper to form a mirror. As yet another way, Patent Document 3 listed below proposes a method according to which, in an organic EL element having a transparent electrode, a first dielectric layer, a light-emitting layer, a second dielectric layer, and a back electrode laid in this order on a transparent, between the transparent electrode and the first dielectric layer, a third dielectric layer is provided that has a refractive index intermediate between theirs. This improves the transmittance with which light comes out, but does not prevent total reflection.
To overcome these inconveniences, Patent Document 4 listed below proposes a method according to which, in the organic EL element mentioned above, a diffraction grating is formed at the interface between the transparent substrate and the first electrode, or at the interface between the first electrode and the light-emitting layer, so that the diffraction grating makes equal to or smaller than the critical angle the emergence angles of the light incident at incidence angles equal to or larger than the critical angle. This enhances the light extraction efficiency.
On the other hand, Patent Document 5 listed below proposes a method according to which, in an organic EL element having a light-emitting layer, a electrically conductive transparent film, and a glass plate provided in this order on a back metal electrode, between the electrically conductive transparent film and the glass plate, a low-refraction member is provided. This enhances the rate of light extraction from the low-refraction member and the glass plate.    Patent Document 1: JP-A-S63-314795 (page 2, FIG. 1)    Patent Document 2: JP-A-H1-220394 (pages 2 and 3, FIG. 1)    Patent Document 3: JP-A-S62-172691 (pages 2 and 3, FIG. 1)    Patent Document 4: JP-A-H11-283751 (pages 3 and 4, FIG. 2)    Patent Document 5: JP-A-2001-202827 (pages 2-5, FIG. 3)